1. Field of the Invention
The invention relates generally to a device structure for up and down conversion of radiation, typically infra-red, wherein low energy wavelengths are up converted to higher energy to improve adsorption by a photosensitive device.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98    U.S. Pat. No. 4,842,960 U.S. Pat. No. 4,891,507 U.S. Pat. No. 6,541,788 U.S. Pat. No. 7,008,559
There is considerable interest in up conversion of infrared to shorter wavelength light, for various applications. The approaches currently described in prior art fall into the following broad categories which take fundamentally different approaches to the problem.
Sandoval [Sandoval_IRPT—04] describes a system for thermal imaging using emission from rare earth doped glass fibers. In this system, infrared radiation from the target is focused onto a rare-earth doped fiber, creating local fluctuations in temperature. The fiber is pumped with 800 nn light giving rise to steady state emission at 520-550 nm. The infrared light impinging transversely on the fiber generates local variations in temperature of the fiber which shifts the spectrum of the emission. In this scheme the target infrared light does not directly take part in the up conversion process, as different to this application.
Chen, U.S. Pat. No. 7,008,559, describes a biological imaging system where rare-earth containing nano-particles are attached or combined within objects of interest and made to fluoresce when irradiated. The instant invention does not require attachment or embedding of a fluorescent material to a target.
Boyd [Boyd-00] and others, using non-linear optical crystals such a LiNiO3 or atomic vapors (Na for example) to sum a pump light energy with a target infrared light energy to up convert for imaging. The up conversion process described in the prior art is via second and third order non linear optical interactions which take place in entirely different materials than those disclosed in the present work.
Another scheme for up conversion is described in [Wu-optics letters 2007] and U.S. Pat. No. 6,541,788. In these methods, infrared light is absorbed within quantum dot structures, to create electrical carriers which then emit light via an adjacent light emitting diode (LED). In this work no LED structure is present; light emission is via atomic transitions in excited rare earth, RE, ions, not band-to-band radiative recombination as in a LED.
Up conversion plates for image intensifiers have been described by Lindemayer in U.S. Pat. No. 4,891,507 and U.S. Pat. No. 4,842,960. The prior art discloses RE materials within a CaS or SrS host, which must be pre-charged from a light source to trap electrons within the material. The present invention neither requires pre-charging, nor does it embody CaS or SrS host materials. Furthermore, the fundamental mechanism for generating the up converted light is fundamentally different, i.e. these materials rely on electrons being transferred to trapping sites by a pre-charge light, with electrons released from these levels by the infrared radiation.
[Auzel-Chem-reviews]—Describes an up conversion scheme where a vitroceramic phosphor material is used as an up converting layer. The present work confines itself to a solid film layer deposited upon a detector, optionally silicon.
[Bloembergen] Describes IR sensing via ESA, where excitation to a first excited state is provided by the target radiation, which then enables ESA of shorter wavelength light. Furthermore, the scheme of Bloembergen requires that a fourth energy level be present so that up converted light may be discriminated from pump light. The instant invention requires no fourth level, and the pump wavelength is discriminated by virtue of it being longer than the cutoff wavelength of the semiconductor detector.
[Auzel-Revue-Phys-Appl] Auzel reports a Yb—Er co-doped vitroceramic pumped at 960 nm for detection of 1550 nm radiation via detection of 660 nm red radiation resulting from up conversion. Auzel does not disclose the up conversion material deposited on a silicon detector; nor does Auzel disclose a pumping wavelength that is out of the detector bandwidth; nor does Auzel disclose an integrated filter structure such as a DBR.
It is well known that certain materials containing rare-earth ions are capable of up converting infrared light to visible light. Typically up conversion is concerned with converting one wavelength of light to a shorter wavelength of light, an example is up converting 1550 nm light to 980 nm light in Erbium doped materials.
Infrared imaging is an important technology especially for defense, security and chemical sensing applications. The high cost of such systems often precludes their use in many commercial, medical and domestic applications where such technology may prove useful. The present invention discloses a method to supplement a conventional CMOS or CCD silicon focal plane array as is found in conventional visible imaging systems, to serve as an infrared imaging system. Furthermore the present invention is not limited to silicon-based imaging systems. Detectors based on GaAs, InGaAs, GaN or other semiconductor systems incorporating an integrated up conversion device may also be envisioned. In such a system, the detector is responsive to sub-band gap radiation through the use of an up conversion device as described herein. In some embodiments an up conversion device may not be integrated directly on the detector surface; it may be a distinct layer of material apart from a radiation detector.